Mesoporous TiO2 Embedded with a Uniform Distribution of CuO

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Mesoporous TiO2 Embedded with a Uniform Distribution of CuO Exhibit Enhanced Charge Separation and Photocatalytic Efficiency Yu-Te Liao, Yu-Yuan Huang, Hao Ming Chen, Kenji Komaguchi, ChiaHung Hou, Joel Henzie, Yusuke Yamauchi, Yusuke Ide, and Kevin C.-W. Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13912 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

Mesoporous TiO2 Embedded with a Uniform Distribution of CuO Exhibit Enhanced Charge Separation and Photocatalytic Efficiency Yu-Te Liao,a Yu-Yuan Huang,a Hao Ming Chen,b Kenji Komaguchi,c Chia-Hung Hou,d Joel Henzie,e Yusuke Yamauchi,e,f Yusuke Idee* and Kevin C.-W. Wua* a

: Department of Chemical Engineering, National Taiwan University. No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan b c

: Department of Chemistry, National Taiwan University, Taipei 106, Taiwan

: Department of Applied Chemistry, Graduate school of Engineering, Hiroshima University, Japan

d

: Graduate Institute of Environmental Engineering, National Taiwan University, Taiwan

e

: International Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), Japan 1-2-1 Sengen, Tsukuba-city, Ibaraki 305-0047, Japan f

: Australian Institute for Innovative Materials (AIIM) & School of Chemistry, University of Wollongong Squires Way, North Wollongong, NSW 2500, Australia ABSTRACT: Mixed metal oxide nanoparticles have interesting physical and chemical properties, but synthesizing them with colloidal methods is still challenging and often results in very heterogeneous structures. Here we describe a simple method to synthesize mesoporous titania nanoparticles implanted with a uniform distribution of copper oxide nanocrystals (CuO@MTs). By calcining a titanium-based metal-organic framework (MIL-125) in the presence of Cu ions we can trap the Cu in the TiO2 matrix. Removal of the organic ligand creates mesoporosity and limits phase separation so that tiny CuO nanocrystals form in the interstices of the TiO2. The CuO@MTs exhibits superior performance for photocatalytic -1 hydrogen evolution (4760 µmol h ) that is >90 times larger than pristine titania. Keywords: Metal-Organic Frameworks (MOFs) • TiO2 • Metal oxides loading • CuO • Photocatalytic hydrogen evolution

TiO2 is a promising photocatalyst for environment and energy applications due to its high photocatalytic activity, low economic cost, and high stability. Charge separation efficiency 1 is the main limitation on photo-catalytic activity in TiO2, thus hybridization with metal and metal-oxide co-catalysts is often 2 used to improve activity. Recently, researchers have explored the impact of co-catalyst size on photocatalytic activities in TiO2 and related semiconductors and observe enhanced performance when co-catalyst particles are crafted at nanoscale 3 dimensions. We hypothesized that the level of homogeneity of the co-catalyst will also play a role in the photoactivity of TiO2. Particularly in the field of mesoporous materials, there has been limited effort to generate materials with uniformly distributed co-catalysts. This is due in part to the limited temperatures and pressures available in conventional synthetic

colloidal methods, where phase separation can play a major 4 role in nucleation and growth of nanocrystals. Metal-organic frameworks (MOFs) are reticular architec5 tures composed of metal ions and organic ligands. Due to their high surface area and tunable surface properties, MOFs have found applications in diverse fields including gas stor6-7 age/separation, catalysis and sensing. MOFs can also be used as templates for more rugged mesoporous materials due to their ordered and abundant metal and carbon sources. By employing pyrolysis and calcination processes it is possible to transform the MOF into carbon and metal oxide nanostructures while maintaining their high surface area and porosity. Different from general procedure for synthesis of porous metal oxide that a template component (e.g. surfactant or co8-10 polymer) is introduced into precursor solution, MOF derived porous metal oxide structures inherit the porous structure from mother MOF. The formation of porous structure without introduction of template during synthesis could prevent the contamination of template residue as well as produce in large scale. We previously demonstrated a novel method to synthesize metal nanoparticles embedded inside MOF nanocrystals. The main feature of this approach is to introduce metal precursors during the synthesis of the MOF superstructure. The MOF grows around the metal ions, resulting in nanocrystals containing a uniform distribution of guest metal precursors. We applied this method to synthesize zeolitic imidazolate frame11 works (ZIF-8) embedded with gold (Au) nanoparticles. After pyrolysis, the MOF was converted into an N-doped nanoporous carbon matrix with a uniform distribution of tiny Au nanoparticles (Au@NCs). These structures showed good performance as water-based reduction catalysts due to the homogeneous distribution of Au nanoparticles and the hydrophilic surface of the imidazole-derived carbon framework.

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Here we describe the synthesis of MOF-derived mesoporous titania embedded with a uniform distribution CuO cocatalyst. In our notation for CuO@MTs(x) the x indicates the mole percentage of CuO precursor in the synthesis solution. Then we show how these structures can be used as catalysts for photocatalytic hydrogen evolution from water. CuO was chosen because hybrid CuOx-TiO2 structures exhibit en12 hanced charge separation and photocatalytic efficiency. The CuO@MTs(0.5) samples exhibited a higher photocatalytic activity than conventional Cu-species-modified TiO2 materials due to the homogeneous distribution of CuO in TiO2 (Table S1). The scheme in Fig. 1a shows the general procedure to form CuO@MT(0.5). 0.5 mole % of a copper precursor was added to the synthetic solution of titanium-based MIL-125 to prepare 13 Cu-doped MIL-125 (called MIL-125(0.5Cu,Ti)). This structure was then calcined at 400°C for 4 h to obtain CuO@MTs(0.5). A more detailed explanation of the synthesis and characterization of MIL-125 and MIL-125(xCu,Ti) is located in the Supporting Information (Fig. S1 to S7). The original MIL125(0.5Cu,Ti) MOF formed 2 µm-sized tablets (Fig. S4c). Calcining the structure removed the terephthalic acid organic lig-

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CuO nanoparticles. Fig. 1C shows nanoparticles with lattice fringes consistent with CuO embedded in the titania matrix (Fig. 1c). Elemental mapping clearly shows that Cu species are uniformly distributed in the CuO@MT particle. X-ray diffraction (XRD) shows that CuO@MTs(0.5) are composed of anatase TiO2 (Fig. 1d). It is likely that most of CuO exists in the form of tiny clusters because no diffraction peaks consistent with copper species (e.g. CuO, Cu2O and Cu) could be observed in XRD. The N2-adsorption/desorption isotherms of CuO@MTs(0.5) (Fig. 1e) have a type IV isotherm that is typical for mesoporous materials with the specific sur2 face area of 63 m /g and a pore size of 4 nm. Similar XRD, SEM and N2 adsorption/desorption data were obtained for CuO@MTs(x) samples synthesized with different amounts of copper precursor (Fig. S5 to S11). The amount of CuO incorporated in the CuO@MTs(x) material can be measured with inductively coupled plasma (ICP) spectroscopy (Table S2). Our results show that the Cu precursors were incorporated almost quantitatively in the products (i.e. 0.5 mole% CuO in CuO@MTs(0.5)), except CuO@MTs(1) (0.86 mole%). This observation indicates that the MIL-125 nucleates homogeneously in the DMF/methanol solution. Interactions between copper, titanium and terephthalic acid (BDC) assemble in the solution (Fig. S8); however, when the amount of Cu precursor is higher than 0.5 mole%, some fraction of the copper nitrate chelates with the BDC, forming Cu-BDC dimers or trimers rather than joining with the titanium-BDC. The chemical state of copper in CuO@MTs(0.5) was examined with X-ray absorption spectroscopy (XAS). The results of Cu K-edge X-ray absorption near-edge structure (XANES) and its first derivative are plotted in Fig. 2a and 2b. Various copper species in different chemical states are added for reference. CuO has a pre-edge peak at 8977 eV in addition to an intense peak at 8984 eV (Fig. 2b). These features correspond to the dipole-forbidden 1s to 3d transition and the dipole-allowed 1s 14 to 4p transition that are indicative of tetrahedral CuO. Cu metal and Cu2O have intense peaks at 8978 and 8979 eV and 15 lack the pre-edge peak. The spectrum of CuO@MTs(0.5) in0 + dicates that the Cu species neither Cu nor Cu . Rather, the presence of the pre-edge peak and intense peak at 8978 eV and

Figure 1. (a) Scheme describing the synthesis of Cu-doped MIL-125 (MIL-125 (xCu,Ti)) intermediate. Calcining the sample creates a mesoporous TiO2 matrix embedded with CuO nanoparticles (CuO@MTs(x)). Characterization of CuO@MTs(0.5): (b) SEM image, (c) HRTEM image inserted with EDS-mapping of Cu element, (d) XRD pattern and (e) N 2 adsorption/desorption isotherm inserted with pore size distribution. ands, causing the structure to shrink and resulting in ~1 µm tablets composed of CuO@MTs(0.5) (Fig. 1b). Transmission electron microscopy (TEM) of CuO@MTs(0.5) shows the porous structure of titania substrate, and its uniform contrast that indicates there are no large Cu particles that might indicate extensive phase separation of the Cu species (Fig. S5). High-resolution transmission electron microscopy (HRTEM) revealed that the CuO@MTs(0.5) samples were riddled with

Figure 2. Characterization of CuO@MTs(0.5) and various reference Cu species with different states by XAS. (a) The XANES spectra and (b) their corresponding first derivatives. (c) EXAFS spectra and (d) their corresponding Fourier transformed EXAFS spectra.


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8988 eV proves that the Cu species is CuO embedded in titania (Fig. S12). Furthermore, the intense peak of CuO@MTs(0.5) had a positive energy shift (~4eV) compared to our reference tetrahedral CuO sample. This energy shift 2+ could be attributed to the Cu being distorted by the octahe4+ 16 4+ dral Ti . Ti in the TiO2 lattice has octahedral symmetry with a shorter bond length between Ti-O than Cu-O in CuO 17 2+ lattice. The results suggest that a fraction of the Cu ions are 4+ doped into the titania matrix, occupying Ti sites. The normalized extended X-ray absorption fine structure (EXAFS) analysis of CuO@MTs(0.5) shown in Fig. 2c revealed that the local environment of the Cu species in CuO@MTs re3 sembles CuO. The Fourier transform (FT) of the k c(k) EXAFS data for CuO@MTs(0.5) exhibited a similar interatomic radius as CuO, with a slight difference in the outer shell radius of Cu-

Figure 3. UV-vis absorption (a) and fluorescence (b) spectra for various CuO@MTs(x). Fluorescence spectra were excited at l = 400 nm. ESR spectra of CuO@MTs(0.5) (c) and CuO@MTs(1) (d) before (top) and after (bottom) exposure to light. O (3.1 Å). This indicates that the CuO nanocrystal is very small 17 in CuO@MTs(0.5). The chemical state of copper in CuO@MTs(x) was also analyzed with X-ray photoelectron + spectroscopy (XPS). The result from XPS suggested the Cu state (Fig. S13) which is opposite from our findings with HRTEM and XAS showing CuO. Hashimoto et al. found a similar phenomenon in their measurements of copper in titanium with X-ray absorption near edge structure (XANES) and XPS that the state of Cu was different in XANES and XPS. Their results also suggested different chemical states of copper was caused by the sample being modified in the high vacuum en18 vironment of XPS and simultaneous exposure to X-rays. We further analyzed CuO@MTs(0.5) with temperature programming reduction (TPR) to realize the state of copper (Fig. S14). Compared with pristine MTs, CuO@MTs(0.5) had a CuO reduction peak around 580 K without an apparent overlapping 19 2+ Cu2O reduction peak. Another peak at 550 K came from Cu 20 coordinated inside the TiO2 lattice, which supports with the 16 shifted peak in Fig. 2b (i.e. 4 eV larger than tetrahedral CuO). This result confirms that the appearance Cu2O in XPS is due to the crucial working environment in XPS analysis (i.e. exposure to X-ray for 80 min under high vacuum chamber at ~2.5E9 torr). Water vapor adsorption can estimate the hydrophilicity of porous surfaces depending on the surface silanol groups

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in order to assess the location of CuO. As shown in Table S3, the MTs and CuO@MTs(0.5) had similar hydrophilicity (i.e. ratio of water to nitrogen), suggesting that CuO was embedded in the framework rather than immobilized on the mesopores via grafting with the surface titanol groups. Optical analyses were performed on CuO@MTs(x) to show potential of CuO@MTs(x) as effective photocatalysts. Fig. 3a shows UV-vis spectra of MTs and CuO@MTs(x). Our pristine MTs have an absorption spectrum typical of titania, with an absorption onset at l~400 nm. CuO@MTs(x) had an absorption shoulder in the l=400-450 nm region and enhanced absorption in the l>650 nm region. Similar spectral features 2+ were observed for TiO2 particles grafted with Cu and clusterlike CuO on TIO2; the former can be assigned to electron 22 transfer from the MTs valence band to Cu(II) or band gap narrowing of MTs due to coupling with CuO (see Fig. S15 and 23 Table S4). The latter can be attributed to the Cu(II) d-d tran18 sition, which was not observed in Cu2O-grafted titania. The absorption in both regions was enhanced by increasing the amount of CuO embedded in the MTs. These results confirm that CuO is embedded in MTs and Cu2O shown in Fig. S13 comes from partial reduction of CuO. In the photoluminescence (PL) spectra the MT sample has a PL peak l~453 nm, which weakened with increasing amounts of the embedded CuO (Fig. 3b). This phenomenon is indicative of electron transfer from excited MTs to the em12 bedded CuO. We further explored the electron transfer mechanism with electron spin resonance (ESR) analysis. Fig. S16 shows the ESR spectra of pristine MTs before (top) and after (bottom) exposure to light from a solar simulator oper4+ ated at 77K. After exposure to light the MTs revert part of Ti 3+ to Ti , which is formed by trapping the photo-excited electrons. The ESR spectra of CuO@MTs(0.5) and CuO@MTs(1) (Fig. 3c and 3d) under the same irradiation conditions had 2+ broad signals due to the hyperfine structure of Cu (around 24 310 mT). When suspended in aqueous methanol solution and exposed to light (a condition identical to the photocata2+ lytic reactions below), the Cu signal weakened significantly,

Figure 4. (a) The hydrogen evolution rate of CuO@MTs(x). (b) A Comparison of the hydrogen evolution rate of CuO@MTs(x) with comparable samples prepared with existing methods. In these experiments the amount of CuO was fixed at 0.5 mole% for all samples to enable a fair comparison. 3+

and the Ti signals (330 mT) could not be detected. This indi4+ 2+ cates there is an effective charge transfer from Ti to Cu , + 25 forming ESR-silent Cu . From all the results of optical analyses, we expected that CuO@MTs(x) will have enhanced photocatalytic activity due to the improvement of charge separation efficiency via electron transfer from excited MTs to CuO.



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The photocatalytic performance of CuO@MTs(x) was evaluated for H2 generation from water containing methanol as a sacrificial agent. As shown in Fig. S17, MTs had significantly 26 higher activity than P25 TiO2, a benchmark TiO2. Given that P25 also has enhanced charge separation as determined by the 3+ 27 presence of Ti signals in the ESR spectrum, the higher activity of MTs can be attributed mainly to the larger surface 2 -1 2 -1 area (ca. 47 m g for P25 vs 60 m g for MTs). As expected, CuO@MTs(x) showed remarkably higher activity, with linear hydrogen evolution against irradiation time for this reaction (Fig. S17b). Fig. 4a summarizes the hydrogen generation rate over MTs and CuO@MTs(x). The rate of hydrogen evolution was enhanced up to 90 times when 0.5 mole% of CuO was em-1 bedded (4760 µmol h ) in the material. Additionally, CuO@MTs(1) had lower activity than CuO@MTs(0.5). This is caused by lower effective charge separation efficiency of CuO@MTs(1), as revealed by ESR data (Fig. 3c and 3d, and Ta-

Scheme 1. Proposed mechanism for photocatalytic evolution of hydrogen with CuO@MTs.

2+

ble S5). The intensity of Cu in ESR spectrum (Fig. 3c and 3d) 2+ was quantified and summarized in Table S5. 56% of the Cu + in CuO@MTs(0.5) was reduced into Cu after exposure to light, 2+ + whereas 35% of Cu in CuO@MTs(1) was reduced into Cu . 3+ The disappearance of Ti after exposure to light in Fig. 3d indicates the well charge transfer from titania to CuO. Lower + percentage of Cu in CuO@MTs(1) demonstrate that some of the photo-excited electrons in CuO@MTs(1) are neutralized after transfer from titania to CuO. One could attribute the 2+ lower reduction percentage of Cu in CuO@MTs(1) to the generation of additional recombination centers in CuO as 28 mentioned by Pal. Remarkably, CuO@MTs(x) had a much higher activity than pCuO@MTs prepared by the post surface 3 modification of MTs with CuO via chemisorption method, one of the state-of-the-art CuO-loaded TiO2 (Fig. S18). These two materials have similar surface area, thus the higher activity of CuO@MTs(0.5) is enhanced by virtue of the uniform distribution of CuO (Fig. S18f). Also, CuO@MTs(0.5) had a higher activity than CuO/P25 prepared via chemisorption method (Fig. S19). Hashimoto revealed that atomically iso2+ lated Cu on TiO2 is preferable for interfacial charge transfer 18 (IFCT). Furthermore, the distortion of TiO2 lattice induced 2+ by embedding Cu (Fig. 2b) would reduce the recombination of electrons and holes, resulting in enhancement of perfor29 mance. These results demonstrate the utility of our simple approach to design mesostructured TiO2 with uniformly embedded CuO co-catalysts. The used CuO@MTs(0.5) was analyzed with XRD and compared with pristine CuO@MTs(0.5) as shown in Fig. S20. The characteristic diffraction peak of Cu (43.41°), CuO (38.68°) and Cu2O(36.4°) were not observed in XRD pattern, indicating that the Cu composites did not migrate and aggregate together after photocatalytic reaction. It

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suggests that CuO@MTs is a stable catalyst that the Cu composites anchor tightly in titania structure without apparent aggregation after exposure to light. To examine the electron transfer pathway, we exposed CuO@MTs(1)-containing water/methanol solution to simulated solar light (Fig. S21). The suspension was initially a lightyellow color that eventually turned into a brown color after exposure to simulated solar light. This suggests that Cu2O is forming in the MTs. The as-prepared suspension also tuned brown after exposure to visible light while no H2 could be detected. This shows that electrons transfer from titania into CuO in our system. Scheme 1 shows the proposed mechanism for our system. Photo-excited electrons are transferred to CuO due to a potential difference (CuO is 0.1 V vs NHE and TiO2 is -0.24 V vs NHE). The reduction potential of CuO to Cu2O is 0.6 V vs NHE and that of Cu2O to Cu is 0.47 V vs NHE. The electrons accumulate in CuO and reduce part of CuO into Cu2O, forming Cu2O/CuO@MTs. Since the CB of Cu2O is + more negative than H /H2, Cu2O would donate electrons to 30 protons, forming H2. After donating electrons to protons, Cu2O oxidizes back into CuO, followed by band shift. The photo-excited electrons on MT would again be available to transfer to CuO, followed by similar process for hydrogen evolution. In summary, we have demonstrated a simple and scalable method to generate mesoporous titania implanted with a homogenous distribution of CuO nanocrystals. These mesoporous structures can serve as efficient photocatalysts for photocatalytic hydrogen evolution. Our approach enables the coconstruction of substrate and catalyst simultaneously, providing a better interconnection (i.e. IFCT) between both parts. Our study also demonstrates that catalysts synthesized in situ inside the titania matrix enables better performance than post-synthesized catalyst due to the homogeneous distribution of CuO in TiO2. We think this synthetic procedure is flexible enough to produce a family of mesostructured mixed metal oxide catalysts for enhanced photocatalysis.

ASSOCIATED CONTENT Supporting Information The detail of chemicals and experiments are described in supporting information. The characterization of MIL-125, MIL-125 derivatives, CuO@MTs(x), post-synthesized catalyst are shown in supporting information, too. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author Kevin C.-W. Wu [email protected] Yusuke Ide [email protected]

ACKNOWLEDGMENT We would like to thank the Ministry of Science and Technology (MOST) of Taiwan (104-2628-E-002-008-MY3; 105-2218-E155-007; 105-2221-E-002-003-MY3; 105-2221-E-002-227-MY3;


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105-2622-E-155-003-CC2) and the Aim for Top University Project at National Taiwan University (105R7706) for the funding support.

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Mesoporous TiO2 Embedded with a Uniform Distribution of CuO Exhibit Enhanced Charge Separation and Photocatalytic Efficiency Yu-Te Liao, Yu-Yuan Huang, Hao Ming Chen, Kenji Komaguchi, Chia-Hung Hou, Joel Henzie, Yusuke Yamauchi, Yusuke Ide and Kevin C.-W. Wu


Mesoporous TiO2 Embedded with a Uniform Distribution of CuO

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